The Global Positioning System (GPS) was implemented in the 1970's as a means to provide reliable positioning information for any location on the globe. Since its inception, the GPS has become increasingly employed in a variety of different types of applications that require accurate measurement of location on the surface of the earth. Some of the different applications which make use of information provided by the GPS include geo-location measuring applications, vehicle navigation applications, tracking applications, mapping applications and timing applications.
Data that is sent from GPS satellites can be used by location measuring applications to determine the position on the earth of a data-receiving device. GPS receivers may be useful for personal recreation activities, such as hiking, kayaking, skiing and other activities that may be carried out in remote locations. Location measuring applications may also be used in moving vehicles, such as automobiles and airplanes, to determine their instantaneous location, and thereby assist in navigating to a particular destination. Location measuring applications also find use in the military field, where increasingly more accurate position information is required for targeting and personnel location.
Navigation applications that employ GPS data are becoming more common in moving vehicles, particularly for determining the best path to be taken to reach a desired destination. For example, automobiles may incorporate GPS receivers to determine present location, and use this information in connection with known street layout information to determine the shortest, or most fuel efficient, path to a desired destination. Similarly, aircraft may employ GPS information for navigational purposes, as well as for landing and take-off guidance. In military applications, GPS data is useful in maneuvering blindly, such as at night, or without the aid of lights or other instruments.
Tracking applications employ GPS data to monitor the movement of people and things. For example, the military may employ GPS tracking applications to monitor the movement of troops and equipment. Emergency response systems might use tracking applications to determine the present location of emergency medical response teams, in an effort to minimize the time required to reach a victim at a desired location.
Mapping applications that utilize GPS signals can be used in cartography for creating more accurate maps. Land surveying and marine surveying may also be enhanced by mapping applications that utilize GPS information. In addition, construction and agriculture may both be improved by mapping applications that utilize precision GPS data to accurately align buildings or crops.
Other applications may employ GPS data to determine precise timing, for example to synchronize widely spaced devices. For example, applications such as mobile communications may achieve high levels of timing precision by utilizing the atomic clocks resident on GPS satellites, without incurring the high cost of incorporating such clocks themselves.
From the foregoing, it can be seen that many different applications make advantageous use of the data provided through the GPS. This data can be obtained from any one or more of the twenty-four satellites that currently constitute the GPS constellation. These satellites are placed in orbits such that a minimum of five satellites are in view from every point on the globe at any given time. Many GPS receivers are configured with an almanac, to enable the receiver to determine the present, or expected, location of each of the GPS satellites.
While the GPS data is useful in a variety of different applications, the preciseness of that data is subject to a number of different errors. For example, some of the errors which can affect the data at a GPS receiver include errors in the satellite clocks, satellite orbital ephemeris error, signal propagation delays induced by the ionosphere and troposphere, errors in the receiver clock, receiver noise, and multi-path propagation. The cumulative effect of these various errors can lead to differences of several meters between an actual position and the position indicated by the GPS receiver. These errors also vary with time since the GPS satellite constellation is moving relative to the Earth and the atmosphere is continually changing. While many applications are not sensitive to errors of this magnitude, such as automobile navigation or personnel tracking, other applications may require extremely precise positioning information. For example, in the landing of an airplane on an aircraft carrier, a positioning error of 10 meters could lead to very drastic results. Accordingly, various efforts have been undertaken to minimize the errors that are inherent to GPS data.
One commonly employed approach to reducing errors in GPS positioning data is known as “differential GPS.” In addition to the mobile GPS receiver that is employed to determine a location, differential GPS utilizes a second, stationary GPS receiver. The location of the stationary GPS receiver is precisely known, and therefore can be used to calculate errors in the signals from the GPS satellite. In essence, the stationary GPS receiver operates in the opposite manner from the mobile GPS receivers. Rather than employ timing signals from the GPS satellites to determine location, the stationary GPS receiver utilizes its known location to estimate what the timing signals from the various satellites should be. These estimated timing signals are then compared to the actual timing signals from the satellites, to compute the errors. These computed errors are used to calculate position-correction data, which is transmitted to the mobile receivers over line of sight (LOS) radio data links. At the mobile GPS receivers, the correction data is used to compensate for the errors in the received GPS satellite signals, and thereby provide a more precise determination of location. Differential GPS accuracy is limited to a few meters compared with standard GPS that is accurate to tens of meters.
While differential GPS enhances the accuracy of the Global Positioning System, its applicability is relatively limited. A significant consideration in this regard is the fact the mobile GPS receiver must be located relatively close to the stationary GPS receiver for the correction data to be useful. As the distance between the mobile GPS receiver and the stationary GPS receiver increases, the GPS satellite data errors which occur at their respective locations will differ, for example due to differing atmospheric and/or signal propagation conditions. Consequently, differential GPS systems are only effective in those situations where the mobile GPS receiver operates in an area that is within a few hundred miles of the stationary GPS receiver. At greater distances, the correction data from the stationary GPS receiver is no longer reliable.
This limited effective range of differential GPS restricts its applicability in certain situations. For example, differential GPS would not be available to ships or airplanes in the middle of the ocean. Similarly, in military applications it is not feasible to locate the stationary GPS receivers within a theater of combat. Hence, if the theater of operation is relatively large, differential GPS cannot be employed to locate or track equipment or personnel within its confines. To overcome these limitations in the ability to locate a target, terminal seekers are employed. These devices rely upon an operator to designate and track a target, for purposes of guiding a moving vehicle to the target. However, terminal seekers of this type are quite expensive, and therefore it is desirable to minimize their use. In addition, they cannot be employed in adverse weather, where the ability to designate the target is compromised.
Current differential GPS cannot provide the precision position and velocity information required to reduce weapon system combined delivery errors to the levels achieved by terminal seekers today. This is due to the uncompensated target location errors, or errors in the assumption of the actual target coordinates relative to the navigation solution in the weapon. Current code phase GPS receivers typically provide velocity data derived from Doppler measurements on the LOS ranging signals from each tracked GPS satellite. Typical performance for the highest quality military P(Y) code receivers is in the 10 cm/sec range. (The notation “P(Y)” refers to the encrypted precise form of GPS signal.)
U.S. Pat. No. 5,899,957 to Loomis (henceforth, “Loomis”) discloses a method and apparatus for providing GPS pseudorange correction information over a selected geographic region S with a diameter of up to 300 km with an associated inaccuracy no greater than 5 cm. N spaced apart GPS reference stations (N>4), whose location coordinates are fixed and are known with high accuracy, are provided within or adjacent to a region R. Each reference station receives GPS signals from at least four common-view GPS satellites, computes its own GPS-determined location coordinates, compares these coordinates with its known location coordinates, determines the pseudorange corrections for its GPS-determined location, and transmits these correction signals to a central station located within or adjacent to a region S. The central station retransmits the pseudorange correction signals throughout the region S. A mobile GPS station within or adjacent to the region S has stored within it the coordinates of the GPS-determined last location of that mobile station and the spatial coordinates of K GPS reference stations (K>3) within S that are closest to the last-determined location of that mobile station. The mobile station then computes the differential GPS corrections for the GPS-determined present location of that mobile station.
The system of Loomis is limited in that the pseudorange correction signals retransmitted by the central station are useful only over a limited geographic area. It is therefore desired to provide a system capable of generating and supplying useful correction information to mobile GPS stations over a much greater area, while using significantly fewer reference GPS receivers.